Stereoselective Protonation of Boron Enolates

Due to the different coordinations of acetic acid and trifluoroacetic acid to the boron atom of the enolate 1, two reaction mechanisms result for the protonation of 1. The protonation with acetic acid leads to an enrichment of the (R) isomer (R)-2. while with trifluoroacetic acid (S)-2 is favored. R* = isopinocampheyl.